Polymerization monomers and polymers

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

Bulk Polymerization. Monomer and polymer (with traces of initiator) are the only constituents in bulk polymerizations. Obviously, the monomer must be soluble in the polymer for this type of process to effectively proceed. Bulk polymerization, also called mass or block polymerization, can occur in stirred-tank reactors, or can be unstirred, in which instance it is called quiescent bulk polymerization. The primary difficulty with bulk polymerizations is that as the polymerization proceeds and more polymer is formed, the viscosity increases, thermal conductivity decreases, and heat removal becomes difficult. 

Initially, only initiator free radicals are present in free radical polymerization. Monomer and polymer free radicals begin to form subsequently. However, polymer free radicals are being destroyed in termination reactions... 

In bulk polymerization, also called mass or block polymerization, monomer and polymer (and initiator) are the only components. When only part of the monomer charge is converted to polymer, however, the problems encountered are more typical of the solution method, which is discussed next. We can differentiate between quiescent and stirred bulk polymerizations. Both methods are applied to systems where polymer is soluble in monomer and progressively increases in viscosity with conversion. In quiescent systems, gel formation, corresponding to infinite viscosity, can occur. Because of the heat of polymerization and autoacceleration, the reaction rate is difficult to control. Heat removal is impeded by high viscosity and low thermal conductivity. The removal of traces of unreacted monomer from the final product is difficult because of low diffusion rates. Conversion of all monomer is difficult for the same reason. 

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